The present invention relates to a method and apparatus for cleaning nuclear fuel elements and more particularly to such a method and apparatus for cleaning both external and internal surfaces of the fuel elements forming longitudinal passages therethrough, the method and apparatus being capable of accommodating various types of fuel elements having different arrangements of longitudinal passages extending therethrough.
In the construction and assembly of nuclear reactors, large numbers of fuel elements are employed to form the reactor cores. Each fuel element typically includes an elongated fuel block of hexagonal cross-section with a number of longitudinal passages formed in the block. These passages may include fuel passages for receiving stacks of fuel rods, the fuel passages then being closed during final assembly of the fuel elements. Additional passages within the elements include control rod passages for permitting the insertion of control rods adjacent the fuel rod stacks to permit regulation over the rate of nuclear activity in the fuel elements assembled into a reactor core. The fuel element may also be typically formed with a center hole as well as dowel pins and dowel holes arranged upon opposite end surfaces of the fuel element. The dowel pins and dowel holes insure proper alignment between fuel elements arranged within a reactor core.
In addition, the present invention particularly contemplates the cleaning of nuclear fuel elements adapted for use in high temperature gas reactors. Within such reactors, a gas such as helium forms the heat transfer or coolant medium and is circulated through longitudinal gas flow passages within the various fuel elements. The coolant gas is circulated through the assembled fuel elements within a reactor core by high speed impellers and it is necessary to insure that the gas flow passages within the fuel elements are free from foreign material which might otherwise interfere with the coolant gas circulation system. In particular, such foreign material may tend to impinge upon and damage the impellers or to cause an undesirable deposite or buildup of the material on the impellers.
During preparation of the individual fuel elements, they are first loaded with stacks of fuel rods as described above and then subjected to high temperatures within a in-block curing process. The curing process is particularly intended to cure or harden nuclear fuel containing pitch forming the nuclear fuel rods. However, upon being subjected to the high temperature environment of the process, some of the pitch tends to escape, resulting in the deposition of a light, flaky carbonized pitch residue on all surfaces of the fuel element.
As indicated above, it is necessary to completely remove the pitch residue from all surfaces of the fuel element before the element is further treated and installed within a reactor core. In addition, it is necessary to insure that the various surface dimensions for the fuel elements are accurately restored after the cleaning operation in order to permit a proper intermeshing fit of the fuel elements within the reactor core.
Accordingly, cleaning of the pitch residue from fuel elements has been a particular problem in the past. There has thus been found to remain a need for a method and apparatus for cleaning such fuel elements to assure the complete removal of foreign material such as pitch residue from the fuel element surfaces, particularly those surfaces which form part of the coolant gas circulation system for a reactor core.
Cleaning of the fuel elements is further complicated because of the nuclear fuel contained within the fuel elements during the cleaning operation. It is therefore necessary to conduct the cleaning operation remote from operating personnel, making it difficult to assure that all surfaces of the fuel elements are adequately cleaned.
Further, it is common to employ a single manufacturing facility for the construction and assembly of fuel elements of different sizes and configurations in order to meet the requirements of different reactor applications. It is therefore also contemplated that a method and apparatus for cleaning nuclear fuel elements be adaptable for cleaning various sizes and configurations of fuel elements.